Add pre-commit hook and make format target

I've added the new local.mk to the package sources. While this
should not be needed for the build, it is the simplest solution,
and won't cause many extra rebuilds, because the file won't change
very often.

package.nix

@@ -167,6 +167,8 @@ in {
           ./m4
           # TODO: do we really need README.md? It doesn't seem used in the build.
           ./README.md
+          # This could be put behind a conditional
+          ./maintainers/local.mk
           # For make, regardless of what we are building
           ./local.mk
           ./Makefile